Variable Speed Drive on Motor

Dang, JRaef

I always learn something when you're around. Thanks!

Having had to "initialize" VFDs, what you are talking about looms in front of of you, but you don't really know what the terms mean. There are so many settings for VFDs nowadays, but only a few are really critical for basic applications.

I haven't fully digested your post yet, but I have no doubt that I will have a much better understanding when I do.

There is a typo in there from a faulty keyboard (iPad), it snipped off a big chunk and truncated something in the middle, I apologize for the confusion this may cause. When I have more time I'll try to figure out what went wrong.

Thanks for the detailed response.

I've heard about Vector and Scalar control in the VFD. I know it's quite common with alot of VFD's you buy in todays market, however it is not mentioned in the VFD manual I am buying.

The VFD is a Grundfos CUE VFD, it does talk about open loop control on page 17, but I'm unsure if this is scalar or vector open loop control.

The manual can be found here:

http://pdfcast.org/pdf/grundfos-cue

Polerz,

Because Grundfoss is a PUMP company, the VFDs gthey sell are really only suitable for pump applications, and therefore they are stripped down to save cost. All this means now days is that they are scalar drives (V/Hz or non-vector). That's OK though because like I said, the extra capabilities of vector control are for the most part lost on centrifugal machine applications. The process being controlled is sloppier than even the worst scalar drive by an order of magnetude. Or conversely, using Vector Control on a centrifugal machine does little or nothing towards improving the accuracy of the process of pumping liquids (or moving air).

In this case, their use of the term "open loop" is being done in a different sense. They are not referring to the PWM control algorithm for firing the transistors, they are referring to the PROCESS loop, i.e. what you are trying to control. Open loop in that sense means there is no feed forward control loop in the system. You tell the pump what speed to run, it runs at that speed regardless of what happens in the process. The opposite to that, called a Closed Lop system, is more often described as a PID Control Loop. PID meaning Proportional, Integral, Derivitive. I don't want to go into a lengthy dissertation on PID contol loops here, there are plenty of resources on the web for that. But what they are meaning in this instance is that the VFD has a PID Loop Controller built-in, and you can use it or not use it. If you don't use it, you are in "Open Loop" control. Separate issue.

I hav worked on a few VSDs. You are required to set the max current the max voltage and also your desired HZ. So no matter what the load is the VSD will try to operate the motor within the parameters you provide, if for some reason the load increases the corresponding load amperes would increase while the speed is still maintained.

Dear friend,

In the Induction motors which is being driven by a VSD at 25Hz (half rated speed), it means this is the speed of rotating magnetic field in the motor and the rotor speed is slight less depending upon slip. In the above mention situation when additional torque is required, the motor current will increase to maintain the speed of 25 Hz.

By an additional setting of current limit in VFD, the speed will decrease if the current reaches to motor's full load rated setting.

Dear Mr.polerz

In this particular description of yours, the FLOW remains reduced.

DHAYANANDHAN.S

Phillip to Jraef: You gave a good answer that has generated a new question: when would it be a good idea to specify vector control vs. scalar control in a VFD? I just did a twin 15,000HP compressor station where the drives came with the motor and compressor skid and did not get into it there. I do use VFD's a lot for blower and pumping control, though and would appreciate knowing when it may be better to use the more precise control. Thanks in advance for your answer.

"...new question: when would it be a good idea to specify vector control vs. scalar control in a VFD?"

Specify Vector Control when you truly need high precision of speed or torque, and/or if you want to run at low speeds. As an example, if you use a scalar drive, you will likely get a speed accuracy of +- 1%. With an open loop vector control, you can easily get 0.05% accuracy and with Closed Loop Vector, 0.01%. The same is basically true for torque. Also, vector control can allow getting full accurate torque at nearly zero speed, basically a 1000:1 speed ratio, and closed loop vector can go right down to zero speed. Scalar control is typically good for speeds in a 4:1 ratio, in other words down to 25% speed. Since most centrifugal machines quit working above even that speed, the added capability is pointless.

So if you were to do something like a wire coiling machine, any change in torque might result in the wire being stretched and thinned. So torque precision is paramount and I would use vector control. Or if you have a conveyor that may have to re-start after a power failure where it is fully loaded, and you need MAXIMUM torque capability, a vecotr drive can create full Breakdown Torque (BDT) of 220% in the motor as opposed to Locked Rotor Torque which is 160% of Full Load Torque. That may make the difference in not having to go to a larger motor.

Closed loop vector control is not all that necessary in most applications, but it is needed when you must know that a motor torque value is present even when the motor is not moving. A classic case in point is a hoist. You MUST know that the motor is producing full rated torque at zero speed BEFORE you release the mechanical brake, otherwise you might drop the load.

But again, all that extra accuracy is pointless on something like a centrifugal pump or fan where the process flow itself is going to vary more than that just by a slight difference in temperature. If you are controlling a high pressure PD pump or blower that has a lot of friction at standstill, an open loop vector drive may be beneficial to overcome that friction at startup without overshooting. But for the most part, non-PD pump and air movement can be done very well without vector control.